Electronic paper has long been a goal of the displays industry, and with the advent of E-Ink technology, high quality paper-like monochrome displays have been available on E-book readers for a number of years. However, such displays, whilst providing very low power-consumption at low refresh rates, and excellent sunlight readability, are incapable of displaying high brightness colour or video rate moving images. The release of the iPad® tablet PC from Apple, with a standard transmissive liquid crystal display (LCD) screen (much like that available on a laptop computer, desk-top PC monitor or television) capable of displaying video rate colour images has thrown the concept of electronic paper for portable devices into confusion. Which is more important, efficiency and sunlight readability, or full-colour video-rate capable screens?
The answer to this question is of course to have a display which is capable of both functionalities, a so-called “dual-mode” display. In a first “low-power” mode, the display operates in a bistable configuration. This mode will be used when the display does not need to cope with video-rate content, i.e. low refresh rates, and as such will have low power-consumption due to its bistability. In a second “video-rate” mode, the display operates in a monostable configuration. This mode will be used when the display needs to cope with video-rate content, i.e. high refresh rates, and as such will have higher power consumption than in the “low-power” mode. The power-consumption in the “video-rate” mode may be comparable with today's standard LCD displays. However, such “single-mode” displays would consume the same power even if they were displaying infrequently updated images, and therefore a dual-mode display can offer significant overall power-saving opportunities.
It should be noted at this stage that the use of bistable and dual-mode displays is most relevant when applied to reflective displays, since the power-saving provided by the bistability is more or less irrelevant if the display is back-lit. The ultimate display for portable devices is therefore dual-mode with a high reflectivity in the bright state.
Dual-mode LCDs have previously been suggested but so far (to the best of the authors' knowledge) none have been manufactured. WO 2007/110582A2 (J. F. Stromer et at.; 16 Mar. 2007) describes a cholesteric liquid crystal mode which can be operated in either a bistable or a video-rate mode. WO 2002/103666 A2 (C. Jones; 27 Dec. 2002) describes a method for operating one of the two bistable states of a zenithally bistable liquid crystal display in an analogue fashion for video-rate operation.
However, such displays will always be limited in brightness due to the fact that LCDs generally require a polarizer, and so reflectivity has a theoretical upper limit of 50% (for monochrome displays) and 17% (for RGB colour displays). Of course these values are never quite reached because of the limits imposed by display aperture ratio, which is around 95% at best for LCDs. A more promising technology for reflective displays is that of electrowetting: since there is no polarizer required, the theoretical limit on reflectivity is 100%, aside of course from the aperture ratio, which is currently much lower for electrowetting but still offers a brightness improvement over LCD.
Both bistable and video-rate electrowetting display modes have been proto-typed and demonstrated. US 2007/0127108 A1 (R. Hayes et al.; 7 Jun. 2007) describes an electrowetting display mode that is capable of video-rate operation. U.S. Pat. No. 7,359,108 B2 (R. Hayes et al.; 15 Apr. 2008) and US2009/0046082 A1 (J. Jacobson et al., 19 Feb. 2009) describe electrowetting display modes which are capable of bistable operation. However, so far no dual-mode electrowetting modes have been suggested.